U.S. Pat. No. 4,479,446 discloses an automatic sewing machine system having the capability of automatically identifying the work being processed within the system. The automatic identification is premised on the ability of the system to optically sense binary codes present on the workpiece holders containing D the work to be processed. The binary codes consist of combinations of opaque and reflective encodings which are decoded by the automatic sewing machine system as binary ones and zeroes respectively. This is accomplished by providing a pair of optical sensors which are fixedly mounted in a manner which allows the encodings of the workpiece holder to be registered underneath the optical sensors. The thus mounted optical sensors respond to the amount of light reflected from the encodings so as to thereby identify whether the encodings thereunder are either opaque or reflective.
It has been found that the ability of each of the optical sensors to correctly identify the encoding appearing underneath is dependent on how much variance occurs in the spacing of the encoding from the particular optical sensor. In this regard, workpiece holders that are of approximately the same thickness present encodings to the sensors that are spaced at approximately the same distance from the sensors. This rather uniform spacing between encodings and sensors allows for the sensing circuitry associated with the sensors to be adjusted so as to appropriately respond to the amount of light coming from an opaque encoding versus a reflective encoding. In this manner, the thus mounted optical sensors within the system disclosed in U.S. Pat. No. 4,479,446 adequately sense the amount of light reflected from the encodings so as to thereby identify the encodings appearing on the presented workpiece holders.
It is to be appreciated, however, that workpiece holders used within automatic sewing machine systems may vary significantly in thickness due to the type of work that is being processed within the automatic sewing machine. In this regard, a workpiece holder may consist of one or more leafs each containing a separate overlay of work that has to be sewn to certain work located underneath. An example of such a workpiece holder is illustrated in U.S. Pat. No. 3,988,993, entitled "Pallet for Registering and Securing A Workpiece Which Includes Overlays". The varying thickness of the composite workpiece holder in this patent causes the top encodings to be spaced at varying distances from the fixedly mounted sensors in U.S. Pat. No. 4,479,446. It has been found that the amount of light reflected from such variably spaced encodings can sometimes produce false readings by the fixedly mounted sensors of U.S. Pat. No. 4,479,446. In particular, it has been found that light being reflected from an opaque encoding located at a minimum distance from a sensor could sometimes cause the sensing circuit associated with the sensor to identify the encoding as being reflective if the sensing circuit had been previously adjusted to trigger in response to approximately the same amount of light being received from a reflective encoding located at the maximum distance from the sensor. When the sensing circuit was further adjusted so as to not respond to this amount of light, then the reflected light coming back from the reflective encoding located at the maximum distance was sometimes insufficient to allow the sensing circuit to appropriately identify the encoding as being reflective.